Electronic article surveillance systems track the movement and location of articles, vehicles and the like, and are particularly useful in the detection of theft in retail stores, libraries, etc. These systems employ portal control using either radio frequency (RF) transceivers or magnetometer scanners at store exits to detect the presence (and unauthorized removal) of a passive RF transponder or magnetized tag. U.S. Pat. No. 4,746,830, which issued May 24, 1988 and which is hereby incorporated by reference into this specification, discloses a coded Surface Acoustic Wave (SAW) transponder for use in such a system. In that system, pulse-modulated RF signals are transmitted by a transceiver, or interrogator, and received by the transponder via its antenna. The signals in the transponder are impressed across bus electrodes and converted by way of piezoelectric action into a SAW at a launching IDT. The SAW propagates on the surface of a crystal substrate composed of LiNbO.sub.3, and then is received by a number of interdigital transducers (IDTs) at different points (and at different times) along the surface of the substrate. The IDTs typically are spaced a constant distance apart along the bus electrodes. The SAW is then converted to a voltage signal at each IDT and radiated back, via the bus electrodes and antenna, as a reply to the interrogator. Further, at prescribed locations along the substrate there are connected or unconnected IDTs, thus producing binary (0 or 1) reply code bits. This SAW transponder is also referred to as a "smart tag", and is used in a way similar to the way bar codes are used in supermarkets.
The conventional smart tag uses a substrate composed of LiNbO.sub.3 crystal, which is very expensive. Accordingly, one object of the present invention is to replace the LiNbO.sub.3 substrate by a substrate composed of an inexpensive low loss material, such as silicon or glass. A piezoelectric copolymer film overlaid on the surface of silicon or glass (each of which is non-piezoelectric) can generate a surface acoustic wave, but any such wave will be extremely attenuated (100-1000 dB) because of high mechanical loss in the polymer. However, the present inventor has proposed the use of bulk waves, which can be efficiently generated with IDTs. The above-referenced '830 patent discusses the use of bulk waves in a bulk acoustic wave (BAW) piezoelectric resonator for detecting field disturbance responses to roadside oscillators. See column 2, lines 48-62. However, the '830 patent dismisses such a device as having an unwieldy size, excessive expense, short range, slow processing speed and limited code capacity. Thus a further object of the present invention is to provide a relatively inexpensive bulk acoustic wave transponder that avoids these shortcomings. The present invention achieves these goals.